Concrete blocks are frequently used for constructing retaining walls. Rectangular blocks have been stacked to form a retaining wall and then back filled on one side to form a terrace. The backfill can exert a sufficient force on the retaining wall blocks to cause the stacked blocks to move out of their original position. In some cases, the blocks are stacked to form a stepped wall so that the front face of each row or tier of blocks is offset slightly behind the front face of the adjacent lower row of blocks. Further, a raised lip has sometimes been formed along the upper edge of the block adjacent the front face to prevent the stepped blocks from sliding out of position, as shown in Arvai et al. U.S. Pat. No. 5,791,827. In order to eliminate this problem, retaining wall blocks have sometimes been formed with a groove extending along a bottom surface parallel to an exposed face of the block and a raised portion is formed on a top surface to extend parallel to the front face. The groove on a stacked block receives the raised portion on an adjacent lower block for maintaining alignment of the front faces of the stacked blocks. Such an arrangement is shown, for example, in Forlini U.S. Pat. No. 5,647,185. Retaining wall blocks also may be formed with round knobs on the top surface which fit into a groove on the bottom surface of a stacked block to permit forming a curved retaining wall, as shown in Wagenaar U.S. Pat. No. 5,337,527. Concrete blocks manufactured for retaining walls have generally not been used for constructing freestanding walls due to the lack of stability and frequently unattractive appearance of one or more of the exposed sides of the blocks.
Another application for concrete blocks and wall panels is to construct freestanding walls. In one type of freestanding wall, large precast wall panels are secured between steel I-beams which are set into a concrete foundation. Such walls have been used, for example, as sound barriers separating highways from residential and business areas. Most existing concrete blocks have not been suitable for constructing freestanding walls due to their rough appearance and lack of stability when stacked. It has been difficult to cast concrete blocks which can interlock to form a strong freestanding wall and which have a pleasing appearance on all exposed sides.